1. Field of the Invention
The present invention generally relates to image forming apparatuses and, in particular, to an image forming apparatus that conveys a medium to be recorded by air suction.
2. Description of the Related Art
As image forming apparatuses such as printers, facsimile machines, copiers, plotters, and multi-task machines having plural such functions, known ink jet recording apparatuses of a liquid ejection recording type use a recording head that ejects, for example, ink liquid droplets. The image forming apparatus of this type ejects ink droplets from the recording head onto a sheet during conveyance to perform image formation (used synonymously with recording, printing, and imaging). Further, an object on which ink droplets are ejected is not limited to a paper but includes an OHP sheet, etc., which is also referred to as a medium to be recorded, a recording medium, a recording sheet, a recording paper, etc. Examples of the image forming apparatus include a serial-type image forming apparatus in which the recording head ejects liquid droplets to form an image while moving in a main-scanning direction and a line-type image forming apparatus using a line-type head in which a recording head ejects liquid droplets to form an image without moving.
Note that in the embodiments of the present invention, the “image forming apparatus” of the liquid ejection recording type refers to an apparatus that ejects liquid droplets onto a medium such as paper, a thread, a fiber, a fabric, leather, metal, a plastic, glass, wood, and a ceramic so as to perform image formation. Further, the “image formation” refers to forming on a medium not only relevant images such as characters and graphics, but also irrelevant images such as random patterns (i.e., liquid droplets are just ejected onto a medium). Further, the “ink” is not particularly limited to one called ink so long as it turns into liquid at the time of ejection, but includes a DNA sample, a resist, a pattern material, etc. Further, the “image” is not particularly limited to a planer image, but includes an image on an object formed in three dimensions, and an image formed by three-dimensionally molding a figure. Further, the “image forming apparatus” is not particularly limited to a liquid ejection recording type, but includes one that performs image formation using an electrophotographic method. In the embodiments of the present invention, the image forming apparatus of a liquid ejection type is used.
As a conveyance unit (mechanism) that conveys a sheet as a medium to be recorded in such an image forming apparatus, there is known one that has plural suction holes formed in a conveyance belt or a platen member for guiding the sheet and suctions a chamber or a negative chamber provided at the back surface of the conveyance belt or the platen member by a suction unit such as a suction fan, so that the sheet is air-attracted to the conveyance belt and the platen member and conveyed.
In such a conveyance unit using the air suction, a suction force to the sheet is reduced as the opening areas of the suction holes formed in the conveyance belt and the platen member increase. Therefore, it is likely that the tip end part and the rear end part of the sheet float.
For addressing this problem, for example, Patent Document 1 proposes a sheet conveyance unit in which an air chamber provided at the back surface of a platen member is divided into plural pieces of air chambers in a sheet conveyance direction, suction fans are provided so as to correspond to the air chambers, and a negative pressure is generated only in selected ones in any of the desired air chambers. Further, Patent Document 2 proposes a sheet conveyance unit in which a conveyance belt is divided into plural pieces of conveyance belts in a direction orthogonal to a sheet conveyance direction (i.e., sheet-width direction), air chambers respectively divided in the sheet-width direction are provided at the back surfaces of the conveyance belts, one suction fan and valves that open and close suction paths for connecting the air chambers to the suction fan are provided, and only selected ones of any of the desired air chambers is operated in accordance with a sheet size. Further, Patent Document 3 proposes an ink jet recording apparatus provided with a unit that opens and closes suction holes formed in a platen member in accordance with a sheet width in a sheet-width direction.
Patent Document 1: JP-B-3690182
Patent Document 2: JP-A-5-107969
Patent Document 3: JP-A-2004-098319
As disclosed in Patent Document 2, the air chamber is divided into the plural pieces of air chambers in the sheet-width direction, and the occurrence of a negative pressure in the air chambers is controlled by the one fan and the valve provided in the respective suction paths. According to this configuration, the sheet conveyance unit becomes complicated in its configuration, and the suction fan serving as a source for generating a negative pressure is commonly used. Therefore, the occurrence of a negative pressure (occurrence of a suction force) cannot be finely controlled so as to correspond to the cases of the air chamber where all the suction holes are sealed by the sheet, the air chamber where some of the suction holes are sealed by the sheet, and the air chamber where the suction holes are not sealed by the sheet at all.